Extruded pipes made from polyolefin polymers are well known for a variety of industrial applications. Typically they are used in the building industry for domestic (e.g. potable) water pipes, radiator pipes, floor-heating pipes and for similar applications in ship building etc. Such pipes can also be used as district heating pipes and as process pipes in the food industry etc. Other applications include the conveyance of gaseous fluids and slurries.
The polyolefin used in extruded pipes is often crosslinked, as this provides a number of advantages. Such advantages include, but are not limited to, long term stability including oxidation resistance, which assists in meeting current codes and standards for potable water applications, flexibility in installation including a “memory effect”, etc. Crosslinked polyethylene (PEX) is commonly used for plastic pipes. There are several varieties of PEX that utilize a number of different crosslinking chemistries and processing technologies. Various PEX grades further contain other additives such as antioxidants and/or stabilizer packages in different concentrations and combinations. Three known varieties of PEX for pipe applications are PEX-a, PEX-b, and PEX-c.
In the PEX-a process (“Engel Method”), the cross-linking is induced by peroxide under the influence of heat and high pressure. The resultant PEX-a composition is crosslinked through carbon-carbon bonds to form the cross-linked polymer network. The PEX-a crosslinking process occurs in the melted stage, as opposed to the primary crosslinking processes for PEX-b and PEX-c. The primary reaction is the formation of free radicals upon decomposition of the peroxide, which has to be present by definition for PEX-a, and subsequently, the free radical abstracts hydrogens from the PE polymer chains. The latter gives new carbon radicals, which next combines with neighboring PE chains to form stable carbon-carbon bonds, i.e., crosslinks. The crosslinking, which is considered to be homogeneous and uniform for PEX-a, gives degrees of crosslinking (typically referred to as CCL) in the range of 70-90% for practical applications. Requirement for CCL is to be above 70% for PEX-a as defined in ASTM International Standard for Crosslinked Polyethylene (PEX) Tubing, F 876-10 (approved Feb. 1, 2010). The PEX-a process may therefore be used to produce good quality pipes.
In the PEX-b process, the crosslinking is induced by moisture and heat over extended pre-determined times typically conducted in a “Sauna atmosphere”. The most commonly used methods are referred to as the Sioplas (two-steps), and the Monosil (one step) methods, respectively. In the Sioplas method, a silane, such as for example a vinylsilane is grafted to a HDPE resin prior to pipe extrusion. In the Monosil method, a silane is blended with the HDPE resin during pipe extrusion. In both methods, which are chemically different in the pre-crosslinking steps, the fundamental principle for the actual crosslinking are practically identical, i.e., the crosslinking occurs in a secondary post-extrusion process that is accelerated by a combination of heat and moisture. The latter combination is the active “reagent”, which is involved in the primary hydrolysis and condensation reaction. In principle, the extruded pipe is exposed to hot water and a steam bath. A fundamental difference to PEX-a, is that for PEX-b, the resultant crosslinks are not between carbon-carbon bonds, but instead, oxygen-silicon covalent bonds (siloxane “bridges”) are formed. In comparison with PEX-a, the crosslink density (CCL) are somewhat lower for PEX-b (65-70%), and the crosslinking is also less uniform.
In the PEX-c process, the crosslinking is commonly referred to as a “cold” method. In the PEX-c process, no chemicals are needed in order to facilitate the crosslinking process, but instead high energy electron beam (EB) irradiation is utilized to create the free radicals necessary for the hydrogen abstraction and subsequent crosslinking to take place. The high energy electron beams are non-selective, i.e., chemical bonds are cleaved in an un-controlled fashion. The latter has the consequence of creating side reactions, together with the reaction aimed for, i.e., the crosslinking of HDPE. The crosslinking density for PEX-c is typically in the 70-75% range, and caution has to be taken with irradiation time since a too long exposure may give discolored products and/or brittleness. PEX-c has been successfully used for many years despite the somewhat challenging production conditions.
Another possible crosslinking process is ultra violet (UV) curing, i.e., photoinduced crosslinking, where a pipe formulation comprising a combination of a polyolefinic polymer such for example polyethylene, a photoinitiator, a co-agent, and a stabilizer package, is exposed to UV radiation to form a crosslinked polymer. In the case where polyethylene is utilized, the final product is a PEX pipe. UV curing is generally considered to be a “green” and environmentally friendly technology, since no solvents are used in the process and no emission of volatile chemicals takes place.
One major challenge that occurs with all extruded pipes used for drinking water applications is the potential issue with leaching of the various additives from the polymer pipe matrix. The various additives which include initiators, stabilisers, co-agents, processing aids, antioxidants, etc. may leach from the polymer matrix over time and can become available to contaminate the fluid contents flowing within the pipe. This problem is a particular issue in cases such as drinking water applications and industry standards exist which quantify the allowable safe levels of leaching of materials from the pipe over a period of time for such applications. The various additives are required to be present in the pipe when manufacturing it in order to facilitate processing of the pipe when extruding the raw material polymer and also to ensure structural integrity and resistance to ageing etc. of the finished pipe. At the same time, the very presence of these materials presents a challenge since these materials may leach from the polymer matrix over a period of time.
The use of plastics pipes in drinking water applications is a challenge as described above. One difficulty is the requirement to find the right stabiliser or combination of stabilizers that gives sufficient long-term stability. One measure of this stability is chlorine resistance according to the standard ASTM F2023. At the same time, the stabilizers, anti-oxidants, photoinitiators, co-agents, and other additives, should not generate too high levels of remaining residuals in the final pipe products, which would make it impossible to meet the standards required for drinking water pipes (in North America this is the NSF 61 standard). However, we have found a novel combination of a specific processing method, i.e., co-rotating twin screw technology and photo-induced crosslinking, which if combined with the novel chemical approach presented in the present invention, produces crosslinked polyolefinic pipes that meets and exceeds the current standards applied for plastic pipes utilized for drinking water applications, such as crosslinked polyethylene (PEX).
A stabiliser package is typically needed to ensure the pipes have practical utility. However, stabilisers also have a tendency to leach from plastics pipes over a period of time. Stabilisation of thermoplastic polymers is usually accomplished by melt blending with one or more stabilisers. In this way a heterophase polymer/stabiliser system is formed, which may be best described as a physical dispersion of a low molecular weight stabiliser in a polymer matrix. The vast majority of commercial stabiliser compounds have very different chemical structure from that of the non-polar host thermoplastic polymer. For this reason, the compatibility of various conventional stabilisers with polyolefins is generally poor, thereby leading to migration i.e. leaching of admixed stabilisers across the boundary of the polyolefin with neighbouring fluids, liquids, gases or solid materials. This loss of stabiliser substantially shortens the lifetime of the polyolefin. Of more concern is the fact that the migration of stabilisers into drinking water (potable water) can also have unpredictable and potentially toxic effects on consumers. We have previously developed a stabiliser package that is suitable for use in drinking water pipes, as is described in WO 2010/138816.
A method for studying stabiliser migration involves immersing the pipe in boiling water with subsequent measurement of the oxidation induction time (OIT) level, which gives an indication of how much active stabiliser is remaining in the pipe and measures how easily the stabiliser is able to leach out of the pipe wall.
Similarly, other additives such as crosslinking agents are required in order to ensure the structural integrity of the pipes in the senses of both their immediate ability to withstand pressure from fluid being transported within, and to ensure their overall long-term performance. The long-term performance of plastics pipes is typically evaluated using the Standard Extrapolation Method (SEM) test of ISO 9080 (e.g. in Europe) or the ASTM D 2837 method (e.g. in North America). These methods involve testing pipes that are pressurised at elevated temperatures and measures the time to burst at different stress levels. Considerable research effort has been focused on so-called stage III ruptures, which take place when the stabiliser package has ceased to be effective or if the degree of crosslinking is insufficient.
EP 0 490 854 B1 describes the use of double screw extruders in combination with UV irradiation to produce crosslinked polyethylene pipes such as those intended for hot water applications. This document discloses specific photoinitiators for achieving crosslinking to enable the fast processing of polymeric materials. A series of benzophenone derivatives is disclosed which are said to be compatible with polyethylene. However, this document does not address the issue of leaching of such materials from the polymer matrix.
The polymer materials of EP 0 490 854 B1 may be prepared using a twin screw extruder. However, the patent is more concerned with the nature of the photoinitiators and does not actually describe the features of the extruder other than its ability to mix and extrude material. The line speeds achievable with the process claimed in that patent are also quite low and were in the range of 1 m/min or less. This is not ideal for a commercial process.
It is apparent that known extruded pipes and methods of making such pipes are subject to a number of limitations. There is therefore a need for new methods of production and/or new combinations of chemical components to improve the methods of production and/or properties of polyolefin pipes.
It is an aim of the present invention to provide materials for forming pipes that can be used in domestic cold and/or hot water application. It is also an aim to provide materials for producing pipes for industrial application. It is an aim to produce pipes which are resistant to the leaching out over time of one or more of the additive components. A further aim is to produce pipes for domestic applications which meet or exceed current standards for one or more of burst strength, pressure resistance, degradation, leaching of additives over time, discolouration, and resistance to chlorine. The present invention satisfied some or all of these aims.